1. Field of the Invention
This present invention relates to a power conversion circuit for driving fluorescent lamps, such as, for example, cold cathode fluorescent lamps (CCFLs) and more particularly to the drive topology of such circuits.
2. Description of the Related Art
Fluorescent lamps are used in a number of applications where light is required but the power required to generate light is limited. One such application is the backlighting for a flat panel computer display, or the like. One particular type of fluorescent lamp is a cold cathode fluorescent lamp (CCFL). CCFL tubes typically contain a gas, such as Argon, Xenon, or the like, along with a small amount of Mercury. After an initial ignition stage and the formation of plasma, current flows through the tube, which results in the generation of ultraviolet light. The ultraviolet light in turn strikes a phosphorescent material coated in the inner wall of the tube, resulting in visible light.
One problem with CCFL tubes is that such tubes do not generate a high level of light output at low temperatures. When these systems are installed where they are exposed to environmental conditions, such as in automobiles, it can take several minutes of operation before the lamp temperature reaches a point to generate an acceptable amount of light output. This makes the backlight systems unusable at low temperatures.
To combat this problem, some manufactures developed "self-heating" lamps. Essentially, these lamps contain two different gases, one optimized to operate at a cold temperature and a second optimized at a normal operating temperature. At low temperatures, the first gas glows and provides an enhanced light output. After the lamp warms up, the second gas takes over and provides the light output. Although these self-heating lamps provide some improvement, the light output at low temperatures is still below the desired range.
Further, it is often desired to control the brightness of the backlight systems Even at low lamp temperatures, it may desirable to have a controllable level of brightness. Dimming of conventional backlight systems is accomplished by adjusting the amplitude of the current. However, the precision of dimming available with amplitude control is limited. Using amplitude controlled dimming, most CCFL tubes are limited to a 3:1 dimming range. Further, decreasing the current amplitude at low temperature may not be feasible.
Because CCFL lamps are installed in a variety of locations, the type of input signal received by a backlight system may vary. The input signals, which may control a variety of functions of the lamp, including but not limited to power on and off, brightness control, contrast control, or the like, may be a digital control signal or a DC voltage. Previously, separate input circuits were necessary depending on the type of input signal to be used.
A power conversion circuit is needed which permits an increased brightness level at low temperatures. Further, the power conversion circuit should be capable of accepting either digital or analog inputs.